The use of inkjet printers for printing information on a recording media is well established. Printers employed for this purpose may be grouped into those that emit a continuous stream of fluid droplets, and those that emit droplets only when corresponding information is to be printed. The former group is generally known as continuous inkjet printers and the latter as drop-on-demand inkjet printers. The general principles of operation of both of these groups of printers are well recorded. Drop-on-demand inkjet printers have become the predominant type of printer for use in home computing systems, while continuous inkjet printers find major application in industrial and professional environments.
Continuous inkjet printers typically have a print head that incorporates a supply system for ink fluid and a nozzle plate with one or more ink nozzles fed by the ink fluid supply. A gutter assembly is positioned downstream from the nozzle plate in the flight path of ink droplets to be guttered. The gutter assembly catches ink droplets that are not needed for printing on the recording medium.
In order to create the ink droplets, a drop generator is associated with the print head. The drop generator influences the fluid stream within and just beyond the print head by a variety of mechanisms discussed in the art. This is done at a frequency that forces thread-like streams of ink, which are initially jetted from the nozzles, to be broken up into a series of ink droplets at a point within a vicinity of the nozzle plate.
Means for selecting printing drops from non-printing drops in the continuous stream of ink drops have been well described in the art. One commonly used practice is that of electrostatically charging and electrostatically deflecting selected drops. A charge electrode is positioned along the flight path of the ink droplets. The function of the charge electrode is to selectively charge the ink droplets as the droplets break off from the jet. One or more deflection plates positioned downstream from the charge electrodes create an electric field which deflects a charged ink droplets either into the gutter or onto the recording media. In some systems, the droplets to be guttered are charged and hence deflected into the gutter assembly and those intended to be printed on the media are not charged and hence are not deflected. In other systems, the arrangement is reversed, and the uncharged droplets are guttered, while the charged droplets are printed. Continuous inkjet printers employing these electrostatic droplet separation means are referred to as electrostatic continuous inkjet print heads.
In high quality inkjet printing, it is desirable to provide small and uniform ink droplets which may be accurately printed on the media. Accordingly, it is desirable to provide a high degree of accuracy with respect to the size and placement of the individual nozzle channels in an array of nozzle channels on the nozzle channel plate. Further, it is desirable that the pressurized fluid delivered through these nozzle channels be provided similar flow conditions from nozzle to nozzle.
An array of inkjet nozzle channels is typically fed by ink from a fluid reservoir. The construction of these reservoirs can lead to dissimilar flow conditions at the inlets of various individual nozzle channels within the array. Dissimilar flow conditions between the inlets to the nozzle channels in the array, can arise from a number of factors. Dissimilar ink flow into the inlets can be caused by differing flow components in the direction perpendicular to flow into the nozzle channels, called cross flow, or parallel to flow in the nozzle channels, called axial flow. Dissimilar flow conditions between nozzle channel inlets can give rise to dissimilar flow in the nozzle channels themselves.
Turbulence arising from ink flow in the reservoir inlet and outlet structures, can also cause variations in fluid flow into the nozzle channel inlets. In the operation of inkjet print heads, cross-flow may arise when ink travels from the reservoir inlet to each of the nozzle channel inlets of the array and/or to the reservoir outlet. The reservoir outlet may be left open during print head operation to maintain a flow of ink which aids in the removal of undesirable particulates that can lead to sedimentation and nozzle clogging. With or without an open reservoir outlet, reservoir inlet geometry and nozzle channel geometry can lead to differential ink flow and turbulence at one or more nozzle channel inlets. Dissimilar flow conditions are likely to occur in inkjet print heads in which nozzle channels located at the ends of an array are subjected to different flow rates or turbulence levels than the nozzle channels located within the central regions of the array.
The ink jetted from any given nozzle channel may retain a memory of the respective ink flow conditions that existed at the inlet of the nozzle channel. The memory of the ink flow conditions results from momentum components and random pressure variations in the fluid and gives rise to some persistence of the initial flow conditions over some distance, depending on the nature of the flow. Given this memory, dissimilar ink flow conditions as between nozzle channel inlets may contribute to a deleterious printing effect referred to as “non-uniform jet pointing”. Non-uniform jet pointing may result in nozzle-to-nozzle variations in the desired trajectories of the jetted fluid streams and their subsequently formed droplets. Other factors including poorly controlled manufacture of the nozzle channels and particulate contamination can contribute to non-uniform jet pointing. Variation in the flow conditions at the nozzle channel inlets may additionally create adverse effects such as jet velocity variability. Variability in jet velocity may lead to variation in the size of the resulting droplets. These effects may lead to additional inaccuracies in droplet placement on the media, and consequently poor printing quality.
This phenomenon is further illustrated by the prior art inkjet print head 10 shown in FIG. 1. Print head 10 includes a fluid reservoir 20 and a plurality of nozzle channels 30. Dissimilar ink flow conditions exist among the various nozzle channel inlets 32. These unequal ink flow conditions may be caused by turbulence or variation in cross-flow or axial flow as ink enters reservoir inlet 40 and travels within reservoir 20 to the various channel inlets 32. The ink can optionally exit reservoir 20 via reservoir outlet 50 and this may contribute to the dissimilar flow conditions that exist between channel inlets 32. The ink in each channel 32 retains a memory of the particular non-uniform flow conditions that exist at its respective channel inlet 32. Channel inlet flow areas with a great deal of non-uniformity will cause greater amounts of jetted stream instability or trajectory variance as shown by jets 60. Jets 60 may be located at the edges of the array. Channel inlet flow areas with a lesser amount of non-uniformity will create a reduced amount of jetted stream instability or trajectory variance as shown by jets 70. Jets 70 may be located in the interior of the array. This variance in jetted stream trajectory or stability between nozzle channels is referred to in the art as non-uniform jet pointing.
In U.S. Pat. No. 5,912,685, Raman discloses a drop-on-demand (DOD) inkjet head in which an “island” of material is located near the entrance of an ink-firing chamber. This island creates multiple ink feed channels for introducing ink to an ink-firing chamber. The prevalent unequal ink flow conditions that exist in the Raman print head are magnified by changing the aspect ratio of the “island” to induce significantly different fluid resistances in each of the multiple ink feed channels. The higher resistance channels help to reduce unwanted cross-talk effects among the nozzles that are adjacent a particular nozzle that is activated.
In European Patent Application EP 1,219,424, Anagnostopoulos et al. describe a print head that also uses “islands” or blocking structures. These blocking structures however are used to impart additional lateral flow components to ink entering an inkjet nozzle. In the Anagnostopoulos et al. device, asymmetric heating is employed to purposely deflect the jetted streams. The asymmetric heating alone may not provide the degree of deflection that is required. The lateral flow components induced by the blocking structures are a factor in increasing the amount of desired stream deflection. Anagnostopoulos et al. disclose the use of dissimilar ink flow conditions to purposely deviate the trajectory of a jetted stream. Thus, the adverse impact that unequal flow conditions have on nozzle-to-nozzle jet pointing is evident. Additionally, in U.S. Pat. No. 6,491,385, Anagnostopoulos et al. disclose a multi-nozzle print head employing asymmetric heating to deflect the jetted streams and the use of ribs or bridges to create ink channels around each of the nozzles. The ribs are used to strengthen the print head and to reduce pressure variations in the ink channels due to low frequency pressure waves.
Small, closely spaced nozzle channels, with highly consistent geometry and placement can be constructed using micro-machining technologies such as those used in the semiconductor industry. Typically, nozzle channel plates produced with these techniques are made from materials such as silicon and other materials commonly employed in semiconductor manufacture. Further, multi-layer combinations of materials can be employed with different functional properties including electrical conductivity.
Micro-machining technologies include etching through the nozzle channel plate substrate to produce the nozzle channels. These etching techniques can include one of, or a combination of wet chemical, inert plasma or chemically reactive plasma etching processes. The materials employed to produce the nozzle channel plates can have particular etching properties that make them suitable for a particular etching process or that can control the etching rate and the etch profile. The micro-machining methods employed to produce the nozzle channel plates can also be used to produce other structures in the print head. These other structures may include ink feed channels and ink reservoirs. Thus, an array of nozzle channels may be formed by etching through the surface of a substrate into a large recess or reservoir which itself is formed by etching from the other side of the substrate.
Micro-machining techniques for the construction of various inkjet head structures have their limitations. To minimize the deleterious effects of substantially unequal flow conditions among nozzle channel inlets, it is desirable to form nozzle channels having uniform diameter and sufficient length to achieve substantially equal flow conditions across the nozzle channels. Such nozzle channels cannot always be readily manufactured by micro-machining techniques. Technologies such as Deep Reactive Ion Etching (DRIE) can be used to etch silicon to form nozzle channels. The difficulty encountered however, is that in order to achieve similar flow conditions among the nozzle channels, typical channel lengths of several hundred microns are required. State of the art inkjet devices typically require small nozzle channel diameters in the order of 10 to 20 microns. Etch processes such as DRIE are incapable of etching such high aspect ratio channels in a timely manner and with consistent uniformity. Additionally, the long channel lengths which help produce the desired flow conditions in such small nozzle channels lead to significant pressure losses over the channel length. It is desirable to minimize the impact of these pressure losses by operating the ink delivery system at relatively high operating pressures in order to achieve desirable jetting velocities.
There is a general desire for flow conditioning methods and means to improve inkjet print heads by reducing dissimilarities in fluid flow conditions across the nozzle channels in an array to minimize non-uniform jet pointing. There is a further desire to provide for an inkjet print head that can be more readily manufactured with the appropriate flow conditioning means.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.